SR71
In your example, if there is enough runway top stop from 165 instead of 135, then there is also plenty of margin to continue from 135 as well. The type of "silly cock-up" I was referring to was someone trying to do the full calculations from the graphs in the AFM and getting it wrong. A friend of mine had a very interesting departure in a 707 after getting Fahrenheit and Celcius mixed up!!
However, you are absolutely right about modern PC's. One of the companies I fly for regularly have just purchased a system that has a worldwide runway and obstacle database and all the performance for all the different aircraft they operate, which is being loaded onto each aircraft's laptop! This is definitely the way forward.
"Aim for the stars and you may just miss the trees"